Constant MIN_LEADER_VERSION_FOR_MIGRATED_MV_WRITES 

const MIN_LEADER_VERSION_FOR_MIGRATED_MV_WRITES: Version;

The oldest leader version against which a replacement-migrated builtin materialized view may write its new persist shard while this environment is still read-only.

Every builtin materialized view reads mz_internal.mz_catalog_raw, so its dataflow only makes progress up to the catalog shard’s frontier. Holding that frontier at the current time is the leader’s job, and leaders only started doing it in v26.17 (PR #35402). Write-enable such an MV against an older leader and it sits at a stale frontier and never reports caught up, which blocks promotion outright instead of merely leaving the collection cold at cut-over. We still support upgrading from before v26.17, so that leader is a real case, not a hypothetical.